Except as provided in AS 42.05.306, a public utility may not, as to service, make or grant an unreasonable preference or advantage to any person or subject any person to an unreasonable prejudice or disadvantage. A public utility may not establish or maintain or provide an unreasonable difference as to service, either as between localities or as between classes of service, but nothing in this section prohibits the establishment of reasonable classifications of service or requires unreasonable investment in facilities.